Actively controlled suspension system with anti-roll control

ABSTRACT

An actively controlled suspension system includes an anti-roll control loop which is responsive to lateral acceleration to be exerted on a vehicle, to adjust suspension characteristics. The control loop employed suspension system is variable of response characteristics to vehicular rolling depending upon the vehicle speed for varying anti-rolling suspension control characteristics. In a range of the vehicle speed, in which the greater loop gain of the control loop, causes significant and unacceptable level of self-induced lateral vibration, the gain is varied to be the smaller value than that in other vehicle speed range. Namely, since the greater gain may affect for causing self-induced lateral vibration in substantially low vehicle speed range, the gain may be adjusted to smaller value while vehicle speed is lower than a given value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an automotive suspensioncontrol system which actively controls suspension characteristics foradjusting vehicular attitude. More specifically, the invention relatesto an actively controlled vehicular suspension system which featuresanti-roll control.

2. Description of the Background Art

The U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, to HirotsuguYAMAGUCHI et al., and assigned to the common assignee to the presentinvention, discloses an actively or positively controlled automotivesuspension system which includes hydraulic cylinder for generatingvariable damping force against relative motion of a vehicular suspensionmember and a vehicle body in bounding and rebounding directions foradjusting suspension characteristics and adjusting vehicle bodyattitude.

Similar type actively controlled suspension systems has been disclosedin the co-pending U.S. patent application Ser. No. 060,911, filed onJune 12, 1987 and assigned to the common assignee to the presentinvention. Further, European Patent First Publication Nos. 02 49 246 and02 49 209 respectively corresponding to co-pending U.S. patentapplications Ser. Nos. 061,368 (filed on June 15, 1987) and 059,888,filed on June 9, 1987, both of which have been assigned to the commonassignee to the present invention. The later-mentioned three inventionsincludes features of anti-roll suspension control for adjusting dampingcharacteristics at respective vehicular wheel for suppressing vehicularrolling motion.

Generally, anti-roll suspension control in this type of activelycontrolled suspension system, is performed in terms of lateralacceleration to be exerted on the vehicle body. In order tosatisfactorily suppress vehicular rolling, it is required to providesufficient gain for a lateral acceleration dependent control loop.However, on the other hand, greater gain in the control loop tends toincrease magnitude of self-induced vibration in rolling direction.Therefore, for minimizing or maintaining the self-induced vibrationmagnitude within an acceptable level, the gain of the control loop hasto be limited. The limited loop gain in the control loop tends to makeanti-roll characteristics of the suspension system unsatisfactory.

On the other hand, it is noted by the inventor of the present inventionthat uncomfortable self-induced vibration tends to occur atsubstantially low vehicle speed range. It is also found that magnitudeof self-induced vibration becomes smaller or self-induced vibration willnot affect for riding comfort at higher vehicle speed.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide an activelycontrolled suspension system which is variable of controlcharacteristics depending upon the vehicle speed.

Another object of the invention is to provide a variable anti-rollcontrol characteristics depending upon the vehicle speed.

In order to accomplish the aforementioned and other objects, an activelycontrolled suspension system, according to the invention, includes ananti-roll control loop which is responsive to lateral acceleration to beexerted on a vehicle, to adjust suspension characteristics. The controlloop employed in the present invention is variable of gain dependingupon the vehicle speed for varying anti-rolling suspension controlcharacteristics.

In the practical control, in a range of the vehicle speed, in which thegreater loop gain of the control loop, causes significant andunacceptable level of self-induced lateral vibration, the gain is variedto be the smaller value than that in other vehicle speed range. Namely,since the greater gain may affect for causing self-induced lateralvibration in substantially low vehicle speed range, the gain may beadjusted to smaller value while vehicle speed is lower than a givenvalue.

According to one aspect of the invention, an anti-roll suspensioncontrol system for an automotive vehicle comprises a suspension systemdisposed between a vehicle body and a suspension member which rotatablysupports a vehicular wheel, the suspension system having a variablepressure chamber filled with a working fluid of controlled pressure, apressure adjusting means, associated with the variable pressure chamber,for adjusting the pressure of the working fluid in the pressure chamber,a first sensor means for monitoring the vehicle speed to producing avehicle speed indicative signal indicative of the vehicle speed; asecond sensor means for monitoring vehicular rolling condition forproducing a rolling magnitude indicative signal, and a control unitreceiving the vehicle speed indicative signal and the rolling magnitudeindicative signal, producing an anti-roll suspension control signalbased on the rolling magnitude indicative signal to control the pressureadjusting means for adjusting suspension characteristics to suppressvehicular rolling, the control unit adjusting response characteristicsof anti-roll suspension control in response to the rolling magnitudeindicative signal depending upon the vehicle speed indicative signalvalue.

According to another aspect of the invention, an anti-roll suspensioncontrol system for an automotive vehicle comprises at least first-leftand second-right suspension systems, each being disposed between avehicle body and a suspension member which rotatably supports avehicular wheel, each of the suspension system having a variablepressure chamber filled with a working fluid of controlled pressure, apressure adjusting means, associated with the variable pressurechambers, for adjusting the pressure of the working fluid in thepressure chambers in a manner mutually independent to the other, a firstsensor means for monitoring the vehicle speed to producing a vehiclespeed indicative signal indicative of the vehicle speed, a second sensormeans for monitoring vehicular rolling condition for producing a rollingmagnitude indicative signal, and a control unit receiving the vehiclespeed indicative signal and the rolling magnitude indicative signal,producing first and second anti-roll suspension control signals based onthe rolling magnitude indicative signal to control the pressureadjusting means for adjusting suspension characteristics in the firstand second suspension systems to suppress vehicular rolling, the controlunit adjusting response characteristics of anti-roll suspension controlin response to the rolling magnitude indicative signal depending uponthe vehicle speed indicative signal value.

According to a further object of the invention, an actively controlledsuspension system for an automotive vehicle comprises a suspensionsystem disposed between a vehicle body and a suspension member whichrotatably supports a vehicular wheel, the suspension system having avariable pressure chamber filled with a working fluid of controlledpressure, a pressure adjusting means, associated with the variablepressure chamber, for adjusting the pressure of the working fluid in thepressure chamber, a first sensor means for monitoring magnitude ofrelative displacement between the vehicle body and the suspension memberto produce a bouncing magnitude indicative signal, a second sensor meansfor monitoring vehicular rolling condition for producing a rollingmagnitude indicative signal, a third sensor means for monitoring thevehicle speed to producing a vehicle speed indicative signal indicativeof the vehicle speed and a control unit receiving the bouncing magnitudeindicative signal, the rolling magnitude indicative signal and thevehicle speed indicative signal, producing a suspension control signalbased on the bouncing magnitude indicative signal value to control thepressure adjusting means for adjusting suspension characteristics tosuppress relative displacement between the vehicle body and thesuspension member, the control unit being responsive to rollingmagnitude indicative signal to modify the suspension control signal forsuppressing vehicular rolling, the control unit adjusting responsecharacteristics of anti-roll suspension control in response to therolling magnitude indicative signal depending upon the vehicle speedindicative signal value.

The control unit detects the vehicle speed indicative signal valuesmaller than a given value to lower the response characteristics. Thecontrol unit maintains the response characteristics at a give constantfirst level while the vehicle speed indicative signal value is heldgreater than or equal to the give value. The control unit is responsiveto vary the response characteristics between the first level and zerolevel according to the vehicle speed indicative signal value while thevehicle speed indicative signal value is held lower than the givenvalue. The control unit is set the given value at a value representativeof substantially low vehicle speed.

The pressure adjusting means comprises a pressurized fluid sourceconnected to the variable pressure chamber and a pressure control valveincorporating an electrically operable actuator, the actuator beingconnected to the control unit to receive the anti-roll suspensioncontrol signal to operate the pressure control valve for adjustingamount of working fluid to be introduced into and removed from thevariable pressure chamber for adjusting damping characteristics of thesuspension system. The control unit includes a gain-controlled amplifierwhich determines the response characteristics, the control unit adjuststhe gain of the gain-controlled amplifier corresponding to the vehiclespeed indicative signal value. The control unit compares the vehiclespeed indicative signal value with a predetermined low vehicle speedcriterion to maintain the gain of the gain-controlled amplifier at aconstant first value when the vehicle speed indicative signal value isgreater than or equal to the low vehicle speed criterion, and lowers thegain corresponding to the vehicle speed indicative signal value which issmaller than the low vehicle speed criterion.

The control unit varies the gain of the gain-controlled amplifier in arange between the first value and zero value depending upon the vehiclespeed indicative signal value. The control unit varies the gain of thegain-controlled amplifier linearly corresponding to the vehicle speedindicative signal value.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic illustration of general construction of anactively controlled suspension system;

FIG. 2 is a sectional view of a pressure control valve assembly to beemployed in the preferred embodiment of the actively controlledsuspension system according to the invention;

FIGS. 3(a) and 3(b) are sectional view similar to FIG. 2, but showingthe condition where a piston in the pressure control valve assembly isshifted downwardly for reducing fluid pressure in a fluid chamber of ahydraulic cylinder in the suspension system and upwardly to increase thefluid pressure in the fluid chamber;

FIG. 4 is a graph showing variation of the fluid pressure in a hydrauliccylinder as controlled by the pressure control, valve assembly of FIG.2;

FIG. 5 is a schematical and diagramatical illustration of a hydrauliccircuit to be employed for the preferred embodiment of the line pressurecontrol of the invention.

FIG. 6 is a schematic block diagram of the preferred embodiment ofsuspension control system of FIG. 1;

FIG. 7 is a graph showing relationship between gain control signal R anda gain K_(y) in a gain-controlled amplifier which adjusts rollsuppressive suspension control characteristics, employed in thesuspension control system of FIG. 6;

FIG. 8 is a graph showing relationship between gain control signal R anda vehicle speed;

FIG. 9 is a graph showing relationship between vehicle speed and gainK_(y) ;

FIG. 10 is a graph showing relationship of gain control signals E_(f)and E_(r) and gain K_(f) and K_(r) in gain-controlled amplifiers whichare cooperative to each other for adjusting pitching suppressivesuspension control characteristics, employed in the suspension controlsystem of FIG. 6;

FIGS. 11(a) and 11(b) are graphs showing relationship betweenlongitudinal acceleration G_(x) and gains E_(f) and E_(r) ;

FIG. 12 is a graph showing another example of relationship betweenlongitudinal acceleration G_(x) and gains K_(f) and K_(r) ;

FIG. 13 is an illustration showing a model to be utilized for discussionabout logic of anti-roll suspension control;

FIG. 14 is a chart showing stable and unstable regions of rollcontrolling gain K_(y) ;

FIG. 15 a chart showing variation characteristics of frequencytransmission coefficient in one cycle of anti-roll control;

FIG. 16 is a flowchart showing a routine for adjusting roll-suppressivesuspension characteristics control gain; and

FIG. 17 is a flowchart showing a routine for adjustingpitching-suppressive suspension characteristics control gain.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, particularly to FIG. 1, a vehicle has foursuspension systems 11FL, 11FR, 11RL and 11RR for respectively suspendingvehicle body 12 on front-left, front-right, rear-left and rear-rightroad wheels 14FL, 14FR, 14RL and 14RR. Each of the front-left,front-right, rear-left and rear-right suspension systems 11FL, 11FR,11RL and 11RR comprises a suspension member 13, such as a suspensionlink, a suspension arm and so forth, and a suspension assembly 15 whichis interposed between the vehicle body 12 and the suspension member 13.The suspension assembly 15 has a hydraulic cylinder 15A which serves asan actuator for generating damping force against bounding and reboundingmotion between the vehicle body and the suspension member, and a coilspring 16.

It should be appreciated that, in the shown embodiment, the coil spring16 is not necessary to damp dynamic kinematic energy and shouldresiliently support only static load to be exerted between the vehiclebody and the suspension member. However, it should be, of course,possible to employ the coil spring which can be strong enough to damppart of dynamic kinematic energy in relative bounding and reboundingmotion of the vehicle body and the suspension member.

The hydraulic cylinder 15A has a hollow cylinder housing 15a filled witha viscous working fluid and a piston 15c sealingly and thrustinglydisposed within the internal space of the cylinder housing to divide thecylinder space into upper and lower fluid chambers 15d and 15e. A pistonrod 15b extends through one end of the cylinder housing 15a. The otherend of the cylinder housing 15a is connected to one of the vehicle body12 and the suspension member 13. On the other hand, the end of thepiston rod 15b is connected to the other of the vehicle body 12 and thesuspension member 13.

The hydraulic cylinder 15A of the suspension assembly 15 is connected toa hydraulic pressure source unit 20 via a hydraulic circuit whichincludes pressure control valve 18. The pressure control valve 18 iselectrically operable and connected to a microprocessor-based controlunit 100. The hydraulic circuit 19 includes a supply line 19s and adrain line 19d. The pressure source unit generally comprises a pressureunit 20 and a reservoir tank 21. The pressure unit 20 is connected tothe reservoir tank 21 to suck the viscous working fluid in the reservoirtank 21 to feed to the pressure control valve 18 via the supply line19s. On the other hand, the drain line 19d is connected to the reservoir21 to return the working fluid thereto

As seen from FIG. 1, a pressure accumulators 22P are communicated withthe upper fluid chamber 15d of the hydraulic cylinder 15A via a pressureline 22B. A throttle valve 22V is inserted between the upper fluidchamber 15d and the pressure accumulator 22P. In the shown embodiment,the throttle valve 22V has a fixed throttling rate.

FIG. 2 shows the detailed construction of the hydraulic cylinder 15A andthe pressure control valve 18. As will be seen from FIG. 2, the hollowcylinder housing 15a is formed with a port 15f communicating the upperfluid chamber 15d to an outlet port 18d of the pressure control valve 18via a communication line 27. Though FIG. 2 does not show clearconstruction, the lower fluid chamber 15e is defined as an enclosedspace and is filled with the viscous working fluid. The pressure of theworking fluid in the lower fluid chamber 15e at an initial position ofthe piston 15c serves as a reference pressure and per se serves asresistance for downward movement of the piston.

The pressure control valve 18 has a valve housing 18A having theaforementioned outlet port 18d, an inlet port 18b and a drain port 18c.Respective inlet port 18b, the drain port 18c and the outlet port 18dare connected to a valve bore 18a defined within the valve housing 18A.A valve spool 19 is disposed within the valve bore 18a for thrustingmovement therein. The valve spool 19 has first, second and third lands19a, 19b and 19c. As will be seen from FIG. 2, the third land 19c hassmaller diameter than that of the first and second lands 19a and 19b.The third land 19c defines a fifth pressure control chamber 18h which isconnected to the drain port 18c via a drain path 18f. An actuator piston22c is also disposed within the valve bore 18a. The actuator piston 22copposes the second land 19b in spaced apart relationship to define asecond pressure control chamber 18i which is connected to the drain port18c via a drain path 18e. An annular pressure chamber 18j is definedbetween the first and second lands 19a and 19b. The pressure chamber 18jis constantly communicated with the outlet port 18d and wherebycommunicated with the upper fluid chamber 15d. On the other hand, thepressure chamber 18j shifts according to shifting of the valve spool 19to selectively communicate with the inlet port 18b and the drain port18c. On the other hand, an pressure control chamber 18k is definedbetween the first and third lands 19a and 19c. The pressure controlchamber 18k is in communication with the outlet port 18d via a pilotpath 18g. A bias spring 22d is interposed between the actuator piston22c and the valve spool 19. The actuator piston 22c contacts with anactuator rod 22a of an electrically operable actuator 22 which comprisesan electromagnetic solenoid. The solenoid 22 comprises a proportioningsolenoid.

In order to increase the supply pressure of the working fluid, the spoolvalve 19 is shifted to the position shown in FIG. 3(A) to increase patharea at a throttle constituted at the inner end of the inlet port 18b bymeans of the land 19a of the spool valve 19. On the other hand, in orderto decrease the supply pressure of the working fluid, the spool valve isshifted to the position shown in FIG. 3(B) to decrease the path area atthe throttle of the inner end of the inlet port 18b and opens the drainport 18 which is normally blocked by means of the land 19b of the spoolvalve.

Construction of the pressure control valves should not be specified tothe construction as illustrated in FIGS. 2, 3(A) and 3(B) but can bereplaced with any appropriate constructions. For example, the pressurecontrol valve constructions as illustrated in European Patent FirstPublication No. 01 93 124, set forth above, can also be employed. Thedisclosure of the aforementioned European Patent First Publication No.01 93 124 is herein incorporated by reference for the sake ofdisclosure.

As seen from FIG. 2, the proportioning solenoid 22 comprises theactuator rod 22a and a solenoid coil 22b. The solenoid coil 22b isenergized by suspension control signal V₃ from the controller 100. Inthe shown embodiment of the pressure control valve, the working fluidpressure p at the outlet port 18d is variable according to thecharacteristics shown in FIG. 4. Namely, when the control valve V₃ asrepresented by the suspension control signal is zero, the pressure atthe outlet port 18 becomes P₀ determined according to a predeterminedoffset pressure P₀. When the suspension control signal value in positivevalue increases, the fluid pressure P at the outlet port 18d increaseswith a predetermined proportioning gain K₁. Namely, by increasing of thesuspension control value V₃, the actuator rod 22a is driven downwardlyin FIG. 2 at a magnitude toward to position of FIG. 3(A) to achieveincreasing of the fluid pressure with the predetermined proportioninggain K₁. The fluid pressure P at the outlet port 18d saturate at theoutput pressure P₂ of the pressure unit 20. On the other hand, when thesuspension control signal value V₃ decreases, the pressure P decreasesto zero to by shifting of the actuator rod 22a toward the direction toFIG. 3(B).

The actuator rod 22a of the proportioning solenoid 22 is associated withthe actuator piston 22c. Contact between the actuation rod 22a and theactuator piston 22c can be maintained by the resilient force of the biasspring 22d which normally biases the actuator piston toward theactuation rod. On the other hand, the spring force of the bias spring22d is also exerted on the valve spool 19 to constantly bias the valvespool downwardly in FIG. 2. The valve spool 19 also receives upwardhydraulic force from the pressure control chamber 18k. Therefore, thevalve spool 19 is oriented at the position in the valve bore at theposition where the downward bias of the bias spring 22d balances withthe upward hydraulic force of the pressure control chamber 18k.

When bounding motion occurs at the suspension member, the piston 15c ofthe hydraulic cylinder 15A shifts upwardly to cause increasing of thefluid pressure in the upper chamber 15d. This causes increasing of thefluid pressure at the outlet port 18d of the pressure control valve 18.As a result, the fluid pressure in the pressure control chamber 18kincreases by the pressure introduced through the pilot path 18g todestroy the balance between the downward bias of the bias spring 22d andthe upward hydraulic force of the pressure control chamber 18k. Thiscauses upward movement of the valve spool 19 against the spring force ofthe bias spring 22d, as shown in FIG. 3(B). As a result, path area ofthe drain port 18c increases and the inlet port 18b becomes beingblocked. Therefore, the fluid pressure in the fluid chamber 15d isdrained through the drain port. Therefore, the increased fluid pressurein the fluid chamber 15d of the hydraulic cylinder 15A can besuccessfully absorbed so that the bounding energy input from thesuspension member will not be transmitted to the vehicle body.

When rebounding motion occurs at the suspension member, the piston 15cof the hydraulic cylinder 15A shifts downwardly to cause decreasing ofthe fluid pressure in the upper chamber 15d. This causes decreasing ofthe fluid pressure at the outlet port 18d of the pressure control valve18. As a result, the fluid pressure in the pressure control chamber 18kdecreases by the pressure introduced through the pilot path 18g todestroy the balance between the downward bias of the bias spring 22d andthe upward hydraulic force of the pressure control chamber 18k. Thiscauses downward movement of the valve spool 19 against the spring forceof the bias spring 22d, as shown in FIG. 3(A). As a result, path area ofthe inlet port 18b increases and the drain port 18c becomes beingblocked. Therefore, the fluid pressure in the fluid chamber 15d isincreased by the pressure introduced through the inlet port. Therefore,the decreased fluid pressure in the fluid chamber 15d of the hydrauliccylinder 15A can be successfully absorbed so that the rebounding energyinput from the suspension member will not be transmitted to the vehiclebody.

Here, since no flow resisting element, such as orifice, throttlingvalve, is disposed between the fluid reservoir 21 and the drain port18c, no damping force against the upward motion of the piston 15c in thehydraulic cylinder 15A will be produced in response to the boundingmotion of the suspension member. Since the damping force exerted on thepiston 15c may serves to allow transmission of the part of boundingenergy to the vehicle body to cause rough ride feeling, the shownembodiment of the suspension system may provide satisfactorily highlevel riding comfort by completely absorbing the bounding and reboundingenergy set forth above.

FIG. 5 shows detailed circuit construction of the preferred embodimentof the pressure source unit to be employed in the suspension controlsystem according to the invention. As set forth, the pressure sourceunit includes the pressure unit 20 which comprises a fluid pump, and isconnected to the reservoir 21 via a suction pipe 201 which is driven bymeans of an automotive engine 200. The outlet of the pressure unit 20,through which the pressurized working fluid is fed, is connected to theinlet port 18b of the pressure control valve 18 via the supply line 19s.A pressure regulating orifice 202 is disposed in the supply line 19s forsuppressing pulsatile flow of the working fluid and whereby regulate theoutput pressure of the pressure unit 20 to be delivered to the pressurecontrol valve 18. A feedback line 19f is connected to the upstream ofthe pressure regulating orifice 202 at one end. The other end of thefeedback line 19f is connected to the upstream of the inlet of thepressure unit 20. Therefore, excessive fluid between the pressure unit20 and the orifice 202 is fed back to the inlet side of the pressureunit.

A pressure accumulator 203 is also connected to the supply line 19s toreceive therefrom the pressurized fluid for accumulating the pressure.An one-way check valve 204 is disposed in the supply line 19s at theposition upstream of the junction between the pressure accumulator 203and the supply line 19s.

A pressure relief line 205 is also connected to the supply line 19s atthe position intermediate between the pressure regulating orifice 202and the one-way check valve 204, at one end. The other end of thepressure relief line 205 is connected to the drain line 19d. A pressurerelief valve 206 is disposed in the pressure relief line 205. Thepressure relief valve 206 is responsive to the fluid pressure in thesupply line 19s higher than a give value to drain part of the workingfluid to the drain line for maintaining the pressure in the supply line19s below the given pressure value.

On the other hand, a shut-off valve 207 is disposed in the drain line19d. The shut-off valve 207 is also connected to the supply line 19s atupstream of the one-way check valve 204 to receive therefrom thepressure in the supply line as a pilot pressure, via pilot line 208. Theshut-off valve 207 is designed to be maintained at open position as longas the pilot pressure to be introduced through the pilot line 208 isheld at a pressure level higher than or equal to a given pressure level.At the open position, the shut-off valve maintains fluid communicationbetween the inlet side and outlet side thereof so that the working fluidin the drain line 19d may flow therethrough to the reservoir tank 21. Onthe other hand, the shut-off valve 207 is responsive to the pilotpressure drops below the given pressure level to be switched intoshut-off position. At the shut-off position, the shut-off valve blocksfluid communication between the drain port 18c and the reservoir tank21.

In parallel relationship to the shut-off valve, a pressure relief valve209 is provided. The pressure relief valve 209 is disposed in a by-passline 210 connecting the upstream side and downstream side of theshut-off valve 207. The pressure relief valve 209 is normally held atclosed position to block fluid communication therethrough. On the otherhand, the pressure relief valve 209 is responsive to a fluid pressure inthe drain line 19d upstream thereof, higher than a set pressure, e.g. 30kgf/cm , in order to establish fluid communication between the upstreamside and downstream side of the shut-off valve to allow the excessivepressure at the upstream side drain line 19d to be drained therethrough.Therefore, the pressure relief valve 209 limits the maximum pressure atthe set pressure. The set pressure of the pressure relief valve 209corresponds to a predetermined offset pressure.

An oil cooler 211 is disposed in the drain line 19d for cooling theworking fluid returning to the reservoir tank 21.

Pressurized fluid supply operation to be taken place by the pressuresource unit as set forth above will be discussed herebelow.

While the automotive engine 200 is running, the fluid pump as thepressure unit 20 is driven. Therefore, the working fluid in thereservoir tank 21 is sucked via the suction pipe 201 and pressurizedthrough the pressure unit 20. The pressurized working fluid isdischarged from the outlet of the pressure unit 20 and fed to thepressure control valve 18 via the supply line 19s including the pressureregulating orifice 202 and the one-way check valve 204. When thepressure control valve 18 in a position of FIG. 3(A), the pressurizedworking fluid passes the pressure control valve and introduced into theupper fluid chamber 15d of the hydraulic cylinder 15. On the other hand,when the pressure control valve 18 is in the position of FIG. 3(B) toblock communication between the supply line 19s and the upper fluidchamber 15d, the line pressure in the supply line increases. When theline pressure in the supply line 19s becomes higher than a set pressureof the pressure relief valve 206 in the pressure relief line 205, theexcessive pressure higher than the set pressure is fed to the drain line19d via the pressure relief valve 206 and thus returned to the reservoirtank 21.

The fluid pressure in the supply line 19s is also fed to the shut-offvalve 207 via the pilot line 208. As set forth, the shut-off valve 207is placed at open position as long as the pilot pressure introducedthrough the pilot line 208 is held higher than or equal to the setpressure thereof. Therefore, fluid communication between the pressurecontrol valve 18 and the reservoir tank 21 is maintained. When thepressure control valve 18 is in the position of FIG. 3(B), the workingfluid is thus returned to the reservoir tank 21 via the drain line 19dvia the shut-off valve 207 and the oil cooler 211.

Since the shut-off valve 207, even at the open position, serves as aresistance to the fluid flow. Therefore, the fluid pressure in the drainline 19d upstream of the shut-off valve 207 becomes excessively higher,i.e. higher than the off-set pressure P₀. Then, the pressure reliefvalve 209 becomes active to open for allowing the excessive pressure ofthe working fluid to flow through the by-pass line 210.

When the engine 200 stops, the pressure unit 20 cease operation. Bystopping of the pressure unit 20, the working fluid pressure in thesupply line 19s drops. According to drop of the pressure in the supplyline 19s, the pilot pressure to be exerted to the shut-off valve 207 viathe pilot line 208 drops. When the pilot line 208 drops below or equalto the set pressure, the shut-off valve 207 is switched into shut-offposition to block fluid communication therethrough. As a result, thefluid pressure in the drain line 19d upstream of the shut-off valve 207becomes equal to the pressure in the upper fluid chamber 15d. Therefore,even when the working fluid leaks through a gap between the spool valve19 and the inner periphery of the valve bore 18a, it will not affect thefluid pressure in the upper fluid chamber 15d.

This is advantageous to maintain the suspension characteristics of thesuspension systems irrespective of the engine driving condition.

In order to perform control for adjusting suspension characteristicsdepending upon the vehicle driving condition, the microprocessor-basedcontrol unit 100 is provided. The suspension control system includingthe control unit 100 is illustrated in FIG. 6. The control unit 100,illustrated in FIG. 6, includes a microprocessor 101 which comprises anarithmetic circuit 102, a memory 104 and input/output unit 106. Avehicle speed sensor 108 is provided for monitoring the vehicle speed toproduce a vehicle speed indicative signal DV. The vehicle speed sensor108 employed in the shown embodiment of the suspension control system,is designed to produce a pulse train having a frequency proportional tothe vehicle speed as the vehicle speed indicative signal DV. A lateralacceleration sensor 110 is also provided for monitoring lateralacceleration to be exerted on the vehicle to produce a lateralacceleration indicative signal Gy. The control system also includes alongitudinal acceleration sensor 112 for monitoring longitudinalacceleration exerted on the vehicle for producing a longitudinalacceleration indicative signal Gx. Furthermore, vertical accelerationsensors 114FL, 114FR, 114RL and 114RR are provided for monitoringvertical acceleration at respective front-left, front-right, rear-leftand rear-right suspension systems 11FL, 11FR, 11RL and 11RR to producevertical acceleration indicative signals. Hereafter, the accelerationsensor 114FL monitoring vertical acceleration at the front-leftsuspension system 11FL will be referred to as "FL vertical accelerationsensor". Similarly, the acceleration sensor 114FR monitoring verticalacceleration at the front-right suspension system 11FR will be referredto as "FR vertical acceleration sensor"; the acceleration sensor 114RLmonitoring vertical acceleration at the rear-left suspension system 11RLwill be referred to as "RL vertical acceleration sensor"; and theacceleration sensor 114RR monitoring vertical acceleration at therear-right suspension system 11RR will be referred to as "RR verticalacceleration sensor". The vertical acceleration indicative signalsproduced by respective FL, FR, RL and RR vertical acceleration sensors114FL, 114FR, 114RL and 114RR will be hereafter referred to respectivelyas "FL vertical acceleration indicative signal Gz_(FL) ", "FR verticalacceleration indicative signal Gz_(FR) ", "RL vertical accelerationindicative signal Gz_(RL) "and "RR vertical acceleration indicativesignal Gz_(RR) ".

The vertical acceleration sensors 114FL, 114FR, 114RL and 114RR may, inpractice, comprise a strain gauge or piezoelectric sensor mounted at thetop end portion of the suspension struts at respective front-left,front-right, rear-left and rear-right wheels. The practical constructionof the vertical acceleration sensor has been disclosed in the co-pendingU.S. patent application Ser. No. 120.964, filed on Nov. 16, 1987. Thedisclosure of the above-identified U.S. patent application Ser. No.120,964 is herein incorporated by reference for the sake of disclosure.

The lateral acceleration sensor 110 is connected to a gain-controlledamplifier 116. The gain controlled amplifier 116 is also connected tothe input/output unit 106 of the microprocessor 101 via adigital-to-analog (D/A) converter 118 to receive therefrom a gaincontrol signal R. The longitudinal acceleration sensor 112 is connectedto gain-controlled amplifiers 120F and 120R.

Respective FL, FR, RL and RR vertical acceleration sensors 114FL, 114FR,114RL and 114RR are connected to integrators 122FL, 122FR, 122RL and122RR. Each of the integrators 122FL, 122FR, 122RL and 122RR integratescorresponding one of the FL vertical acceleration indicative signalGz_(FL), FR vertical acceleration indicative signal Gz_(FR), RL verticalacceleration indicative signal Gz_(RL) and RR vertical accelerationindicative signal Gz_(RR). The integrators 122FL, 122FR, 122RL and 122RRare connected to amplifiers 124FL, 124FR, 124RL and 124RR which hasfixed gain The amplifiers 124FL, 124FR, 124RL and 124RR are connected toinverted input terminal of adders 126FL, 126FR, 126RL and 126RR. Theadders 126FL and 126FR have another inverted input terminals connectedto the amplifiers 120F and 120R. On the other hand, the adders 126RL and126RR have non-inverted input terminals connected to the amplifier 120.The outputs of the adders 126FL, 126FR, 126RL and 126RR are connected tonon-inverting input terminals of another adders 128FL, 128FR, 128RL and128RR. The adders 128FL and 128RR have inverting input terminalsconnected to the gain-controlled amplifier 116. On the other hand, theadders 128FR and 128RL have to the non-inverting input terminalsconnected to the gain-controlled amplifier 116. The output terminals ofthe adders 128FL, 128FR, 128RL and 128RR are connected to theproportioning solenoids 22 in the pressure control valves 18. Therefore,the solenoids 22 adjusts the position of the pistons 15c and wherebyadjust the pressure in the upper fluid chambers 15d of respectivehydraulic cylinders 15A in respective suspension systems 11FL, 11FR,11RL and 11RR according to the outputs of the adders 128FL, 128FR, 128RLand 128RR serving as the suspension control signals S.sub. FL, S_(FR),S_(RL) and S_(RR). The control signals output from the adder 128FL willbe hereafter referred to as "FL control signal S_(FL) "; the controlsignals output from the adder 128FR will be hereafter referred to as "FRcontrol signal S_(FR) "; the control signals output from the adder 128RLwill be hereafter referred to as "RL control signal S_(RL) "; thecontrol signals output from the adder 128RR will be hereafter referredto as "RR control signal S_(RR) ".

As will be appreciated, the vertical acceleration indicative signalsGz_(FL), Gz_(FR), Gz_(RL) and Gz_(RR) are utilized for bounding control,in which relative displacement of the vehicle body and the suspensionmembers is monitored. Suspension characteristics at respectivefront-left, front-right, rear-left and rear-right suspension system areadjusted for suppressing bouncing. For this purpose, the integrators122FL, 122FR, 122RL and 122RR integrates the values of verticalacceleration indicative signals from respectively corresponding verticalacceleration sensors 114FL, 114FR, 114RL and 114RR to output the valuesrepresentative of relative displacement of the vehicle body andsuspension members in bounding and rebounding direction from apredetermined initial position.

The lateral acceleration sensor 110, the gain-controlled amplifier 116and the adders 128FL, 128FR, 128RL and 128RR introduces the rollingsuppressive factor in the suspension control signals S_(FL), S_(FR),S_(RL) and S_(RR). Namely, in general, the gain-controlled amplifier116, the front-left and rear-right suspension systems are controlled inopposite to that in the front-right and rear left suspension systems forsuppressing vehicular rolling.

The gain of the gain-controlled amplifier 116 is adjusted by the gaincontrol signal R from the microprocessor 101. The gain control signal Rmay be a voltage signal having a voltage level representative of adesired gain in the gain-controlled amplifier 116. The gain-controlledamplifier 116 in the shown embodiment, is provided a gain variationcharacteristics to linearly vary the gain Ky according to variation ofgain control signal value R, as shown in FIG. 7. Namely, as will be seenfrom FIG. 7, the gain Ky is a function (a×R: a is constant) of the gaincontrol signal value R. On the other hand, the gain control signal valueR is set in a form of table which is looked up in terms of the vehiclespeed V. Variation characteristics of the gain control signal value R isexperimentally determined and is designed to avoid unstable range wheregreater gain tends to cause self-induced lateral vibration, as shown inFIGS. 8 and 9. The saturation point V₀ across which gain control signalvalue R variation characteristics changes is experimentally determined.In practice, the saturation point V₀ is set approximately 10 km/h.

On the other hand, the longitudinal acceleration sensor 112, thegain-controlled amplifiers 120F and 120R and the adders 126FL, 126FR,126RL and 126RR are so arranged to introduce pitching suppressing factorin the suspension control signals S_(FL), S_(FR), S_(RL) and S_(RR) tobe output to the actuators. The gain of the gain-controlled amplifiers120F and 120R are adjusted by gain control signals E_(f) and E_(r)produced by the microprocessor 101 and fed to respectively correspondingamplifier via D/A converters 130F and 130R. Since the pitchingsuppressive suspension control requires opposite control of suspensioncharacteristics at front and rear suspension systems, polarity of inputsfrom the amplifiers 120F and 120R to the adders 126FL and 126FR aredifferentiated to that of the adders 126RL and 126RR. Therefore, whenpitching suppressive suspension control is taken place to stiffen orharden front suspension systems, the rear suspension systems arecontrolled to soften the suspension characteristics, and when the frontsuspension systems are soften in pitching suppressive control, the rearsuspension systems are harden.

The gain K_(f) and K_(r) of the gain-controlled amplifiers 120F and 120Rare variable depending upon the gain control signals E_(f) and E_(r)input from the microprocessor 101 via the D/A converters 130F and 130Raccording the linear characteristics shown in FIG. 10. In the shownembodiment, the gain control signals E_(f) and E_(r) are variable of thelevels depending upon the longitudinal acceleration Gx to be detected bythe longitudinal acceleration sensor 112. Namely, when the longitudinalacceleration Gx is increased across zero, the gain control signal levelsE_(f) and E_(r) increase by a given magnitude, as shown in FIG. 11(a).According to variation of the gain control signal level E_(f) and E_(r),the gains K_(f) and K_(r) of the gain-controlled amplifiers 120F and120R varies correspondingly as shown in FIG. 11(b).

By controlling the gains K_(f) and K_(r) as set forth above and asillustrated in FIG. 11(b), the absolute value of gain K_(f) whichdetermines the pitching-suppressive suspension characteristics in thefront suspension systems becomes greater than the absolute value of thegain K_(r) which determines the pitching-suppressive suspensioncharacteristics in the rear suspension systems during nose dive motionwhere the longitudinal acceleration is positive. On the other hand,during counter-action after nose dive, the absolute value of gain K_(r)becomes greater than the absolute value of the gain K_(f). This equalizethe magnitude of pitching motion between the front and rear suspensionsystems. Therefore, suppression of nose dive can successfully achieved.

On the other hand, variation characteristics of the gains K_(f) andK_(r) can be modified as illustrated in FIG. 12. As will be seen fromFIG. 12, the gain variation characteristics is alternated to that inFIG. 11(b). Such variation characteristics may be adapted for thevehicle having suspension characteristics to cause substantiallywinding-up during acceleration. Variation characteristics of the gainsof the gain-controlled amplifiers 120F and 120R may not be specified tothe shown characteristics and can be adjusted depending upon thevehicular suspension characteristics or attitude variation tendency.

Here, the fundamental idea of the present invention of rollingsuppressive suspension control with avoidance of the self-inducedlateral vibration will be discussed with reference to the model shown inFIG. 12. In the shown model, a vehicular body 80 as a sprung mass issuspended on a suspension arms 82 which constitute unsprung mass withvehicular wheel and suspension assembly. The lateral acceleration sensor110 is mounted appropriate position on the vehicle body. The verticalacceleration sensors 114L and 114R are mounted at the top of respectivesuspension assembly. Lateral component of rolling energy which causingrolling motion of the vehicle is thus monitored by the lateralacceleration sensor 110. On the other hand, vertical component of therolling energy is monitored by the vertical acceleration sensors 114Land 114R. The lateral acceleration indicative signal of the lateralacceleration sensor 110, which representative of the lateral componentof the rolling energy, is amplified by a roll control gain Ky. On theother hand, the vertical acceleration indicative values of the verticalacceleration sensors 114L and 114R, which represents vertical componentof the rolling energy, are amplified with a bounding control gains K₃.The amplified lateral acceleration indicative signal and the verticalacceleration indicative signals are added to derive roll suppressivesuspension control signals S_(L) and S_(R). The value of the rollingsuppressive control signals S_(L) and S_(R) are fed to hydraulic systemH(s), i.e. the actuator 22.

It is assumed that the condition of the model of FIG. 13 are as follows:

[SPECIFICATION]

M is a weight of sprung mass;

J is a rolling inertia moment;

K is a spring coefficient of suspension;

L is a treat;

Hr is a roll center height;

H₉ is a gravity center height

K_(L) is a lateral strength of tire;

K_(V) is a vertical strength of tire;

C is an equivalent absorption of cornering power of tire, whichequivalent absorption represents cornering force of the tire and thus isvariable depending upon the vehicle speed;

[CONSTANT]

H₁ is a height of the lateral acceleration sensor;

[VARIABLES]

θ_(ys) is a roll angle of the sprung mass;

θ_(yu) is a roll angle of the unsprung mass;

x is a vertical displacement of the sprung mass;

y is a lateral displacement of the sprung mass;

z is a vertical displacement of the unsprung mass;

w is a displacement of traction point.

The cornering force C of the tire can be illustrated by:

    C=θ.sub.SL ×(V.sub.1 /V.sub.2)

where

θ_(SL) is a slip angle;

V₁ is lateral motion speed;

V₂ is longitudinal rotation speed.

As will be appreciated. the value C becomes infinity (θ_(SL) ×0/0) whilevehicle is stopping. The cornering force decreases from infinityaccording to increase of the vehicle speed. Therefore, in substantiallylow vehicle speed range, the cornering force of the tire issubstantially great. Therefore, during substantially low vehicle speedrange, vehicle is laterally supported merely by the cornering force C ofthe tire. In this vehicle speed range, when the rolling suppressive gainK₁ is substantially big, it tends to destroy left/right balance of theforces to cause self-induced lateral vibration in response tosubstantially small magnitude of lateral acceleration. This means thesuspension condition of the vehicle in regard to the rolling stability,stability factor becomes smaller as increasing the rolling suppressivegain K₁. This unstable range is illustrated in FIG. 14.

On the other hand, since the cornering force C decreases according toincreasing of the vehicle speed, vehicle becomes to be supported inlateral direction by a series system of the reduced cornering force Cand the lateral strength K_(L) of the tire. Therefore, stability factorincreases as shown in FIG. 14.

In a condition where self-induced lateral vibration occurs, vibrationfrequency transmission coefficient for one cycle of vibration ismonitored in a vehicle speed of 5 km/h and 10 km/h. The result of themeasurement is shown in FIG. 15. As will be appreciated, the lateralsuspension system is unstable at the vehicle speed of 5 km/h and becomesstable at the vehicle speed of 10 km/h.

The hereafter discussed is the practical suspension control operation toperform roll-suppressive or anti-roll suspension control with referenceto FIG. 16.

The shown routine is executed with a predetermined regular intervals,e.g. every 20 ms. Immediately after starting execution of the routine,the vehicle speed indicative signal DV is read and counted for a givenperiod of time at a step 1002. The arithmetic circuit 102 is then activeto calculate the vehicle speed data V on the basis of the counted numberof pulses with the given period of time, at a step 1004.

In the alternative, the vehicle speed can also be derived by measuringthe interval of pulses which is inversely proportional to the vehiclespeed.

The vehicle speed data V derived at the step 1004 is compared with apreference data which corresponds to the predetermined vehicle speed V₀across which variation characteristics of the gain of thegain-controlled amplifier 116 varies, at a step 1006. When the vehiclespeed data V is greater than or equal to the V₀ representative referencedata, the gain control signal value R is set at the value R₀ which isthe constant value used while the vehicle speed is higher than thepredetermined speed V₀, at a step 1008.

On the other hand, when the vehicle speed data V is smaller than the V₀representative reference data as checked at the step 1004, the gaincontrol signal value R is derived corresponding to the vehicle speed, ata step 1010. The control signal value R derived at the step 1010 issmaller than the gain control signal value R₀ derived at the step 1008.Namely, as will be seen from FIG. 8, the gain control value R as derivedat the step 1010 varies within a range between zero to R₀ linearlycorresponding to the vehicle speed, i.e. zero to V₀.

After one of the steps 1008 and 1010, process goes to a step 1012 tooutput a gain control signal R to the gain-controlled amplifier 116.

Therefore, response characteristics of anti-roll suspension control insubstantially low vehicle speed range is substantially lowered than thatin relatively high vehicle speed range. This successfully suppressself-induced lateral vibration of the vehicle to assure riding comfortat low vehicle speed range. On the other hand, since the self-inducedlateral vibration can be thus successfully prevented, satisfactorilyhigh response characteristics of anti-roll suspension control can beprovided in relatively vehicle speed range. Therefore, the shownembodiment of the suspension control system may provide substantiallyhigh roll stability without degrading riding comfort at substantiallylow vehicle speed range.

On the other hand, adjustment of pitching suppressive suspension controlis performed according to the routine of FIG. 17. The routine of FIG. 17is also triggered with a given constant interval, e.g. 20 ms.

Immediately after starting execution, the longitudinal accelerationindicative signal value Gx is read at a step 1102. Based on the readlongitudinal acceleration indicative signal value Gx, longitudinalacceleration value G_(LOG) is derived at a step 1104. Based on thelongitudinal acceleration value G_(LOG) derived at the step 1104, thegain control signal values E_(f) and E_(r) are derived according to thecharacteristics illustrated in FIGS. 11(a) and 11(b), at a step 1106.The gain control signals E_(f) and E_(r) for the gain-controlledamplifiers 120F and 120R derived at the step 1106, are output at a step1108.

Since the pitching-suppressive suspension control characteristics can beadjusted by setting the characteristics of variation of the gains of thegain-controlled amplifiers 120F and 120R adapted to the suspensioncharacteristics of the vehicle, appropriate attitude control for thevehicular suspension system can be provided.

Therefore, the present invention fulfills all of the objects andadvantages sought therefor.

While the present invention has been disclosed in terms of the preferredembodiment in order to facilitate better understanding of the invention,it should be appreciated that the invention can be embodied in variousways without departing from the principle of the invention. Therefore,the invention should be understood to include all possible embodimentsand modifications to the shown embodiments which can be embodied withoutdeparting from the principle of the invention set out in the appendedclaims. Namely, though the discussion given hereabove is directed tohydraulic cylinder and hydraulic circuit for producing damping force forcontrolling suspension characteristics in the actively controlledsuspension system according to the invention, it should be possible toapply the same logic in roll-suppressive and/or pitching suppressivesuspension control for hydropneumatic suspension, pneumatic suspensionand so forth. Furthermore, though the shown embodiment discloses thesuspension control system which perform both of roll-suppressive andpitching-suppressive control, it is construct the suspension system toperform only roll-suppressive control.

What is claimed is:
 1. An anti-roll suspension control system for anautomotive vehicle comprising:a suspension system disposed between avehicle body and a suspension member which rotatably supports avehicular wheel, said suspension system having a variable pressurechamber filled with a working fluid of controlled pressure; a pressureadjusting means, associated with said variable pressure chamber, foradjusting the pressure of said working fluid in said pressure chamber; afirst sensor means for monitoring the vehicle speed to producing avehicle speed indicative signal indicative of the vehicle speed; asecond sensor means for monitoring vehicular rolling condition forproducing a rolling magnitude indicative signal; and a control unitreceiving said vehicle speed indicative signal and said rollingmagnitude indicative signal, producing an anti-roll suspension controlsignal based on said rolling magnitude indicative signal to control saidpressure adjusting means for adjusting suspension characteristics tosuppress vehicular rolling, said control unit adjusting responsecharacteristics of anti-roll suspension control in response to saidrolling magnitude indicative signal depending upon said vehicle speedindicative signal value.
 2. An anti-roll suspension control system asset forth in claim 1, wherein said control unit detects said vehiclespeed indicative signal value smaller than a given value to lower saidresponse characteristics.
 3. An anti-roll suspension control system asset forth in claim 2, wherein said control unit maintains said responsecharacteristics at a give constant first level while said vehicle speedindicative signal value is held greater than or equal to said givevalue.
 4. An anti-roll suspension control system as set forth in claim3, wherein said control unit is responsive to vary said responsecharacteristics between said first level and zero level according to thevehicle speed indicative signal value while said vehicle speedindicative signal value is held lower than said given value.
 5. Ananti-roll suspension control system as set forth in claim 2 wherein saidcontrol unit is set said given value at a value representative ofsubstantially low vehicle speed.
 6. An anti-roll suspension controlsystem as set forth in claim 1, wherein said pressure adjusting meanscomprises a pressurized fluid source connected to said variable pressurechamber and a pressure control valve incorporating an electricallyoperable actuator, said actuator being connected to said control unit toreceive said anti-roll suspension control signal to operate saidpressure control valve for adjusting amount of working fluid to beintroduced into and removed from said variable pressure chamber foradjusting damping characteristics of said suspension system.
 7. Ananti-roll suspension control system as set forth in claim 6, whereinsaid control unit includes a gain-controlled amplifier which determinessaid response characteristics, said control unit adjusts the gain ofsaid gain-controlled amplifier corresponding to said vehicle speedindicative signal value.
 8. An anti-roll suspension control system asset forth in claim 7, wherein said control unit compares said vehiclespeed indicative signal value with a predetermined low vehicle speedcriterion to maintain said gain of said gain-controlled amplifier at aconstant first value when said vehicle speed indicative signal value isgreater than or equal to said low vehicle speed criterion, and lowerssaid gain corresponding to said vehicle speed indicative signal valuewhich is smaller than said low vehicle speed criterion.
 9. An anti-rollsuspension control system as set forth in claim 8, wherein said controlunit varies said gain of said gain-controlled amplifier in a rangebetween said first value and zero value depending upon the vehicle speedindicative signal value.
 10. An anti-roll suspension control system asset forth in claim 9, wherein said control unit varies said gain of saidgain-controlled amplifier linearly corresponding to said vehicle speedindicative signal value.
 11. An anti-roll suspension control system foran automotive vehicle comprising:at least first-left and second-rightsuspension systems, each being disposed between a vehicle body and asuspension member which rotatably supports a vehicular wheel, each ofsaid suspension system having a variable pressure chamber filled with aworking fluid of controlled pressure; a pressure adjusting means,associated with said variable pressure chambers, for adjusting thepressure of said working fluid in said pressure chambers in a mannermutually independent to the other; a first sensor means for monitoringthe vehicle speed to producing a vehicle speed indicative signalindicative of the vehicle speed; a second sensor means for monitoringvehicular rolling condition for producing a rolling magnitude indicativesignal; and a control unit receiving said vehicle speed indicativesignal and said rolling magnitude indicative signal, producing first andsecond anti-roll suspension control signals based on said rollingmagnitude indicative signal to control said pressure adjusting means foradjusting suspension characteristics in said first and second suspensionsystems to suppress vehicular rolling, said control unit adjustingresponse characteristics of anti-roll suspension control in response tosaid rolling magnitude indicative signal depending upon said vehiclespeed indicative signal value.
 12. An anti-roll suspension controlsystem as set forth in claim 1, wherein said control unit detects saidvehicle speed indicative signal value smaller than a given value tolower said response characteristics.
 13. An anti-roll suspension controlsystem as set forth in claim 12, wherein said control unit maintainssaid response characteristics at a give constant first level while saidvehicle speed indicative signal value is held greater than or equal tosaid give value.
 14. An anti-roll suspension control system as set forthin claim 13, wherein said control unit is responsive to vary saidresponse characteristics between said first level and zero levelaccording to the vehicle speed indicative signal value while saidvehicle speed indicative signal value is held lower than said givenvalue.
 15. An anti-roll suspension control system as set forth in claim12, wherein said control unit is set said given value at a valuerepresentative of substantially low vehicle speed.
 16. An anti-rollsuspension control system as set forth in claim 11, wherein saidpressure adjusting means comprises a pressurized fluid source connectedto said variable pressure chambers via first and second pressure controllines and first pressure control valves respectively incorporating anelectrically operable actuators, each of said actuator being connectedto said control unit to receive one of said first and second saidanti-roll suspension control signal to operate corresponding one of saidpressure control valve for adjusting amount of working fluid to beintroduced into and removed from said variable pressure chamber foradjusting damping characteristics of the corresponding one of first andsecond suspension systems.
 17. An anti-roll suspension control system asset forth in claim 16, wherein said control unit includes again-controlled amplifier which determines said responsecharacteristics, said control unit adjusts the gain of saidgain-controlled amplifier corresponding to said vehicle speed indicativesignal value.
 18. An anti-roll suspension control system as set forth inclaim 17, wherein said control unit compares said vehicle speedindicative signal value with a predetermined low vehicle speed criterionto maintain said gain of said gain-controlled amplifier at a constantfirst value when said vehicle speed indicative signal value is greaterthan or equal to said low vehicle speed criterion, and lowers said gaincorresponding to said vehicle speed indicative signal value which issmaller than said low vehicle speed criterion.
 19. An anti-rollsuspension control system as set forth in claim 18, wherein said controlunit varies said gain of said gain-controlled amplifier in a rangebetween said first value and zero value depending upon the vehicle speedindicative signal value.
 20. An anti-roll suspension control system asset forth in claim 19, wherein said control unit varies said gain ofsaid gain-controlled amplifier linearly corresponding to said vehiclespeed indicative signal value.
 21. An anti-roll suspension controlsystem as set forth in claim 11, which further comprises third andfourth sensors respectively arranged in the vicinity of said first andsecond suspension systems for monitoring bouncing magnitude to producefirst and second bouncing magnitude indicative signals, and said controlunit receives said first and second bouncing magnitude indicativesignals to derive said anti-roll suspension control signal on the basisof said first and second bouncing magnitude indicative signals and saidrolling magnitude indicative signal.
 22. An actively controlledsuspension system for an automotive vehicle comprising:a suspensionsystem disposed between a vehicle body and a suspension member whichrotatably supports a vehicular wheel, said suspension system having avariable pressure chamber filled with a working fluid of controlledpressure; a pressure adjusting means, associated with said variablepressure chamber, for adjusting the pressure of said working fluid insaid pressure chamber; a first sensor means for monitoring magnitude ofrelative displacement between said vehicle body and said suspensionmember to produce a bouncing magnitude indicative signal; a secondsensor means for monitoring vehicular rolling condition for producing arolling magnitude indicative signal; and a third sensor means formonitoring the vehicle speed to producing a vehicle speed indicativesignal indicative of the vehicle speed; a control unit receiving saidbouncing magnitude indicative signal, said rolling magnitude indicativesignal and said vehicle speed indicative signal, producing a suspensioncontrol signal based on said bouncing magnitude indicative signal valueto control said pressure adjusting means for adjusting suspensioncharacteristics to suppress relative displacement between said vehiclebody and said suspension member, said control unit being responsive torolling magnitude indicative signal to modify said suspension controlsignal for suppressing vehicular rolling, said control unit adjustingresponse characteristics of anti-roll suspension control in response tosaid rolling magnitude indicative signal depending upon said vehiclespeed indicative signal value.
 23. An anti-roll suspension controlsystem as set forth in claim 22, wherein said control unit detects saidvehicle speed indicative signal value smaller than a given value tolower said response characteristics.
 24. An anti-roll suspension controlsystem as set forth in claim 23, wherein said control unit maintainssaid response characteristics at a give constant first level while saidvehicle speed indicative signal value is held greater than or equal tosaid give value.
 25. An anti-roll suspension control system as set forthin claim 24, wherein said control unit is responsive to vary saidresponse characteristics between said first level and zero levelaccording to the vehicle speed indicative signal value while saidvehicle speed indicative signal value is held lower than said givenvalue.
 26. An anti-roll suspension control system as set forth in claim23, wherein said control unit is set said given value at a valuerepresentative of substantially low vehicle speed.
 27. An anti-rollsuspension control system as set forth in claim 22, wherein saidpressure adjusting means comprises a pressurized fluid source connectedto said variable pressure chamber and a pressure control valveincorporating an electrically operable actuator, said actuator beingconnected to said control unit to receive said anti-roll suspensioncontrol signal to operate said pressure control valve for adjustingamount of working fluid to be introduced into and removed from saidvariable pressure chamber for adjusting damping characteristics of saidsuspension system.
 28. An anti-roll suspension control system as setforth in claim 27, wherein said control unit includes a gain-controlledamplifier which determines said response characteristics, said controlunit adjusts the gain of said gain-controlled amplifier corresponding tosaid vehicle speed indicative signal value.
 29. An anti-roll suspensioncontrol system as set forth in claim 28, wherein said control unitcompares said vehicle speed indicative signal value with a predeterminedlow vehicle speed criterion to maintain said gain of saidgain-controlled amplifier at a constant first value when said vehiclespeed indicative signal value is greater than or equal to said lowvehicle speed criterion, and lowers said gain corresponding to saidvehicle speed indicative signal value which is smaller than said lowvehicle speed criterion.
 30. An anti-roll suspension control system asset forth in claim 29, wherein said control unit varies said gain ofsaid gain-controlled amplifier in a range between said first value andzero value depending upon the vehicle speed indicative signal value. 31.An anti-roll suspension control system as set forth in claim 30, whereinsaid control unit varies said gain of said gain-controlled amplifierlinearly corresponding to said vehicle speed indicative signal value.